1. Field of the Invention
This invention relates to a magnetic recording head and a magnetic recording apparatus provided with a spin torque oscillator, suitable for realizing data storage with high recording density, high recording capacity, and high data transfer rate.
2. Background Art
In the 1990s, the practical application of MR (magnetoresistive effect) heads and GMR (giant magnetoresistive effect) heads triggered a dramatic increase in the recording density and recording capacity of HDD (hard disk drive). However, in the early 2000s, the problem of thermal fluctuations in magnetic recording media became manifest, and hence the increase of recording density temporarily slowed down. Nevertheless, perpendicular magnetic recording, which is in principle more advantageous to high-density recording than longitudinal magnetic recording, was put into practical use in 2005. It serves as an engine for the increase of HDD recording density, which exhibits an annual growth rate of approximately 40% these days.
Furthermore, the latest demonstration experiments have achieved a recording density exceeding 400 Gbits/inch2. If the development continues steadily, the recording density is expected to achieve 1 Tbits/inch2 around 2012. However, it is considered that such a high recording density is not easy to achieve even by using perpendicular magnetic recording because the problem of thermal fluctuations becomes manifest again.
As a recording scheme possibly solving the above problem, the “microwave assisted magnetic recording scheme” is proposed. In the microwave assisted magnetic recording scheme, a high-frequency magnetic field near the resonance frequency of the magnetic recording medium, which is sufficiently higher than the recording signal frequency, is locally applied. This produces resonance in the magnetic recording medium, which decreases the coercivity (Hc) of the magnetic recording medium subjected to the high-frequency magnetic field to less than half the original coercivity. Thus, superposition of a high-frequency magnetic field on the recording magnetic field enables magnetic recording on a magnetic recording medium having higher coercivity (Hc) and higher magnetic anisotropy energy (Ku) (e.g., U.S. Pat. No. 6,011,664, hereinafter referred to as Patent Document 1). However, the technique disclosed in Patent Document 1 uses a coil to generate a high-frequency magnetic field, and it is difficult to efficiently apply a high-frequency magnetic field during high-density recording.
Techniques based on a spin torque oscillator are also proposed as a means for generating a high-frequency magnetic field (e.g., US Patent Application Publication No. 2005/0023938, hereinafter referred to as Patent Document 2; US Patent Application Publication No. 2005/0219771, hereinafter referred to as Patent Document 3; and 3. Zhu et al., “Microwave Assisted Magnetic Recording (MAMR)”, Digest of the 18th Magnetic Recording Conference (TMRC), B6, p. 34-35, 2007, hereinafter referred to as Non-Patent Document 1). In the techniques disclosed in Patent Documents 2 and 3 and Non-Patent Document 1, the spin torque oscillator comprises a spin injection layer, a nonmagnetic layer, a magnetic layer, and electrodes. When a DC current is passed through the spin torque oscillator via the electrode, the spin torque generated by the spin injection layer produces ferromagnetic resonance in the magnetization of the magnetic layer. Consequently, a high-frequency magnetic field is generated from the spin torque oscillator.
Because the spin torque oscillator has a size of approximately several ten nanometers, the generated high-frequency magnetic field is localized within approximately several ten nanometers around the spin torque oscillator. Furthermore, the perpendicularly magnetized magnetic recording medium can be efficiently resonated by the longitudinal component of the high-frequency magnetic field, allowing a significant decrease in the coercivity of the magnetic recording medium. Consequently, high-density magnetic recording is performed only in a portion where the recording magnetic field of the main magnetic pole is superposed on the high-frequency magnetic field of the spin torque oscillator, allowing utilization of magnetic recording media having high coercivity (Hc) and high magnetic anisotropy energy (Ku), as proposed in, e.g., Non-Patent Document 1. Thus the problem of thermal fluctuations during high-density recording can be avoided.